1. Field of the Invention
This invention relates to limestone-filled asphalt shingles and methods of use thereof to inhibit algae growth on such shingles exposed to varying humidity. The asphalt shingles employ ceramic-coated inorganic granules, both algae-resistant and non-algae-resistant, described in assignee's copending Ser. No. 07/945,128.
2. Description of Related Art
Roofing granules, both natural and artificially color-coated, find extremely wide use in roofing and siding materials. Important applications are in granular surfaced bituminous roll roofing and asphalt shingles. The granules, which are partially embedded in one surface of asphalt-impregnated and/or asphalt-coated fiber sheet material, form a coating to provide an inherently weather-resistant, fire-resistant, and decorative exterior surface. "Weather-resistant" as used herein includes ultraviolet (UV) radiation-resistant.
Skadulis (U.S. Pat. No. 3,528,842) describes roofing granules which contain an algicide, such as cuprous oxide. As explained by Skadulis, methods have long been known to artificially color roofing granules. This consists primarily of utilizing crushed and screened minerals, in which the granules are coated with a suitable pigment contained in an inorganic matrix or bond. The bond is usually applied in the form of a soluble silicate solution and is insolubized either by heat treatment or a combination of heat treatment and chemical action. This yields a substantially water-insoluble coating which is strongly adhered to the base granule. In carrying out these methods, the pigment is typically uniformly applied to the granular surface with the soluble silicate solution. The soluble silicate solution is converted to an insoluble state by heat treatment and chemical action, as by the addition of clay or an acidic material. This can occur either before, during or after the application of the soluble silicate and pigment mixture onto the mineral granules, in order to obtain a reaction between the clay or acidic material and the alkaline portion of the silicate.
U.S. Pat. No. 3,507,676 (McMahon) describes zinc-compounds which contain algicidal surfacing granules. The roofing granules contain either zinc metal, zinc oxide (ZnO) or zinc sulfide (ZnS), rather than copper-containing compounds patented by Skadulis. McMahon teaches the use of a pigment grade ZnO in either a first or second coating, with concentrations of about 10 grams ("gms") ZnO per kilogram ("kg") of inorganic roofing granules. McMahon noted that ZnO had been previously used in pigmented granules as an insolubilization agent or pigment, but was utilized in quantities insufficient to be useful for long-term algae-resistance.
Lodge et al. (U.S. Pat. No. 3,255,031) describes the use of a small amount of a borate, such as "borax" and the like, in clay-silicate coating compositions. The addition of about 2 gms borate per kg of inorganic roofing granules apparently allows the temperature of the firing kiln to be reduced below about 450.degree. C., and results in a roofing granule which does not need to be treated with pickling agents to insolubilize the coating. Color benefits such as "lightening" of the color of all pigmented granules and "whitening" of dark-pigmented granules are derived from the use of borate-clay-silicate coatings. Lodge et al. further disclose that borates can be used in a single coating process or in the inner or outer coating of a two layer coating system. Skadulis and McMahon also disclose that borates may be used in the processes of making ceramic-coated roofing granules.
In the preparation of ceramic-coated roofing granules it is important that the resulting granule exhibit a "low alkalinity" (defined by a test described herein). A low alkalinity is indicative of the thoroughness of insolubilization of the ceramic coating. It is also important that the resulting granules adhere to bituminous surfaces, such as asphalt, with a limited degree of adhesive failure as opposed to cohesive failure of the asphalt. It would thus be advantageous if ceramic-coated roofing granules could be made to have improved adhesion while exhibiting an alkalinity which is initially low and remains low for an extended period of time. The present invention is drawn to such ceramic-coated inorganic granules, methods of making such granules, and a composite sheet body for roofing and siding which includes such granules.
Other patents which are evident of the state of the art include U.S. Pat Nos. 1,720,708; 2,070,359; 2,091,991; 2,111,131; 2,225,867; 2,981,636; 3,046,188; 3,484,267; 3,494,727; 3,507,676; 3,598,627; 3,752,696; 3,826,825; 3,884,706; 3,888,176; 3,888,682; 3,888,683; 3,888,684; 3,894,877; 3,985,540; 3,998,644; 4,088,810; 4,092,441; 4,359,505; and 4,378,408.
In particular, U.S. Pat. No. 2,111,131 discloses that small but highly effective amounts of zinc oxide may be dissolved in sodium silicate by effecting the dissolution at elevated temperatures and pressures, such as by introducing steam into an autoclave containing zinc oxide and the silicate. U.S. Pat. No. 2,225,867 discloses that the effectiveness of copper oxides (especially cuprous oxide) as an algicide is increased by the presence of zinc oxide. Furthermore, the grade of zinc oxide (i.e. purity, source, and the like) was not a factor in the efficacy of inhibiting algae growth.
Minnesota Mining and Manufacturing Company, St. Paul, Minn. ("3M") has commercialized since 1990 pigmented roofing granules known under the trade designation "Series 7000" including "7050" granules These granules have three insolubilized silicate-clay coatings, the first two coatings each having about 35 gms cuprous oxide per kg substrate inorganic granules, with the third coating having no cuprous oxide but alternatively containing pigments to render desired color. None of the three coatings contain zinc oxide or borate.
Assignee's copending application Ser. No. 07/945,128, filed Sep. 15, 1992, of which the current application is a continuation-in-part, describes roofing granules similar to the afore-mentioned LR-7000 and LR-7070 granules, but incorporating zinc oxide made by the "French process" and a borate in the outer ceramic coating.
Assignee's copending application Ser. No. 07/945,127, filed Sep. 15, 1992, also describes algae-resistant granules similar to the afore-mentioned LR-7000 and LR-7070 granules. The '127 application describes improvements in the color matching of algae-resistant granules and the corresponding non-algae-resistant granules including zinc oxide made by the so-called French process and a borate compound in the third layer of a three layer ceramic coating.
Zinc oxide made by the so-called French process and the so-called American process are discussed in detail in Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 24, 3rd Ed. John Wiley & Sons (New York) pp. 854-863.
The roofing industry continually strives for methods of inhibiting algae growth on asphalt roofing materials which will be effective for prolonged time periods, especially in varying humidity and pH conditions. Currently, algae-resistant granules may be produced having varying levels of copper therein. For example, algae-resistant granules may be produced having "loadings" of 40, 60 , or 120 lbs copper/ton of granules (20, 30, or 60 grams/kilogram), depending on the particular production run. In many instances users of algae-resistant granules select the time required for effective algae inhibition and inform their manufacturer or supplier of this requirement. The manufacturer or supplier may not have 30 gram per kilogram granules in stock but may have 20 and 60 gram per kilogram granules. It would beneficial if manufacturers and suppliers could immediately meet the customer's needs for algae-inhibiting granules using available granules.